Understanding Bending Force, Stripper Force, and Blank Holder Force in Metal Stamping

In the precision-driven world of metal stamping and deep drawing, accurately calculating and managing forces is critical to producing high-quality, consistent parts while protecting tooling and presses. Among the most crucial calculations are those for bending force, stripper force, and blank holder force. These forces form the foundation for die design and the selection of appropriate press capacity. This comprehensive guide delves into the theories, calculations, and practical applications of these forces, essential for engineers and manufacturers in the metal stamping and deep drawing industry.

The Critical Role of Force Calculation in Stamping

Bending operations transform flat sheet metal into angled, channel, or complex forms. The forces involved are not trivial; underestimating them leads to incomplete forming, poor part quality, and rapid die wear, while overestimating results in inefficient, oversized press selection, increasing capital and operational costs. Therefore, mastering these calculations is a cornerstone of efficient manufacturing. As a leader in this field, ChinaCustomStamping is a professional metal stamping and deep drawing manufacturer in China, leveraging precise force analysis to ensure optimal production processes and superior product outcomes.

Part 1: Bending Force – The Core Forming Power

Bending force is the primary force required to plastically deform the metal workpiece over the die. It is the key parameter for selecting press tonnage. Its magnitude depends on numerous factors:

Material properties (tensile strength, thickness)
Bending geometry (width, inner bend radius)
Tooling parameters (die opening, punch radius)
The bending method itself (free bending vs. bottoming/correction bending).

Theoretical calculation is complex due to nonlinear material behavior and friction. Hence, the industry relies on proven empirical or simplified theoretical formulas.

1.1 Free Bending Force

Free bending refers to the process where the punch bends the material without fully coining or calibrating it against the die walls at the bottom of the stroke. The workpiece retains some springback.

For V-Bending:

The free bending force ( $F_{f ree}$ ) is calculated as:

Free Bending Force

Where:

$F_{free}$ = Free bending force (N)
$b$ = Width of the bent part (mm)
$t$ = Material thickness (mm)
$r$ = Inner bending radius of the part (mm)
$σ_{b}$ = Tensile strength of the material (MPa)
$k$ = Safety factor, typically 1.0 to 1.3, accounting for material variations, friction, and work hardening.

For U-Bending (Channel Bending):

The formula adjusts for increased contact and friction:

The higher coefficient (0.7 vs. 0.6) reflects the greater force needed for the U-shape, where two bend corners are formed simultaneously, and material flow is more restricted.

Practical Insight from ChinaCustomStamping: Accurately determining the tensile strength ( $σ_{b}$ ) and the actual, post-springback inner radius ( $r$ ) is vital. Using the minimum radius from the drawing often provides a safety margin in force calculation.

1.2 Correction Bending (Bottoming) Force

In correction bending, the punch forces the workpiece completely into the die, applying intense pressure at the bottom dead center to coin and calibrate the bend angle, virtually eliminating springback. Here, the force is determined not by the bending moment but by the pressure applied over the contact area.

The correction force ( $F_{corr}$ ) is given by:

Fcorr=qA

Where:

$F_{corr}$ = Correction bending force (N)
$A$ = Projected area of the corrected (contact) surface (mm²). This is typically the area of the die face in contact with the part around the bend line.
$q$ = Unit correction pressure (MPa), dependent on material type and thickness.

Table: Unit Correction Force (q) Reference Values

Material	Thickness ≤ 1 mm	Thickness >1-2 mm	Thickness >2-5 mm	Thickness >5-10 mm
Aluminum	10 – 15 MPa	15 – 20 MPa	20 – 30 MPa	30 – 40 MPa
Brass	15 – 20 MPa	20 – 30 MPa	30 – 40 MPa	40 – 60 MPa
10, 15, 20 Steel	20 – 30 MPa	30 – 40 MPa	40 – 60 MPa	60 – 80 MPa
25, 30, 35 Steel	30 – 40 MPa	40 – 50 MPa	50 – 70 MPa	70 – 100 MPa

Critical Note: The correction force is highly sensitive to die adjustment (shut height) and minor variations in material thickness. A small over-adjustment can cause force to skyrocket, risking press or die damage. The table values are guidelines; experience and process control are paramount.

Part 2: Auxiliary Forces – Stripper Force and Blank Holder Force

Successful bending often requires auxiliary forces to control the workpiece.

2.1 Stripper Force and Blank Holder Force ( $F_{Q}$ )

These forces serve to:

Hold the workpiece flat against the die plate (blank holder force) to prevent lifting or shifting during punch entry.
Eject the finished part from the die (stripper force) after bending.
Control material flow, especially in complex or precision bends.

This combined force is typically estimated as a percentage of the free bending force:

Where $F_{Q}$ is the stripper or blank holder force (N).

Selection Guide:

0.3 – 0.5 range: Used for simpler bends or when the primary function is part ejection.
0.6 – 0.8 range: Essential for complex shapes, U-bends, or when precise material control during forming is required to prevent wrinkles or shifts.

At ChinaCustomStamping, we precisely calibrate these forces using adjustable gas springs or nitrogen cylinders within our dies. This ensures part stability and ejection reliability across millions of cycles in our high-volume production runs.

Part 3: Determining Press Tonnage Capacity

Selecting a press with adequate tonnage is a non-negotiable safety and quality decision.

3.1 For Free Bending Processes:

The total required press force must account for both forming and auxiliary actions, which occur simultaneously. The press tonnage ( $F_{p ress}$ ) should be:

This ensures the press can handle the peak load when the bending force and the stripper/blank holder force are both active.

3.2 For Correction Bending Processes:

The force profile differs significantly. The massive correction force ( $F_{corr}$ ) occurs at the very bottom of the stroke, in a distinct phase after initial bending. Since $F_{corr}$ is typically far larger than both the initial free bending force and the stripper force, it becomes the sole governing factor for press selection:

This simplified criterion is safe because the free bending phase does not generate enough force to overload a press rated for the subsequent correction force.

Part 4: Advanced Considerations and Industry Practices

4.1 Friction and Lubrication

The coefficient in bending formulas implicitly includes average friction. Using proper lubricants can reduce bending force by 10-20%, decrease tool wear, and improve surface finish. The choice of lubricant—oil, grease, or dry film—depends on material, severity of bend, and subsequent cleaning processes.

4.2 Springback and Its Force Implications

Springback is the elastic recovery of the material after bending, affecting the final angle. While it doesn’t directly change the peak bending force calculation, compensating for it often involves strategies like overbending or correction bending, which directly impact the required force (as seen in the high $q$ values).

4.3 Multi-Stage and Progressive Die Considerations

In progressive dies, multiple operations (bending, punching, drawing) may occur in one press stroke. The total press tonnage must be the sum of all simultaneous forces. Force sequencing and die design become critical to avoid peak load overlaps that exceed press capacity.

4.4 The Role of Modern Simulation (FEA)

While empirical formulas are indispensable, Finite Element Analysis (FEA) software allows for virtual testing. It provides visualizations of material flow, stress distribution, and accurate force predictions for complex geometries, reducing prototyping costs and time. Leading manufacturers integrate these tools for advanced part development.

Conclusion: The Synergy of Theory and Precision Manufacturing

Understanding bending force, stripper force, and blank holder force is not merely an academic exercise but a daily practice in efficient, safe, and high-quality metal stamping. From the basic formulas for V-bending to the nuanced selection of correction pressures, each calculation step directly influences tooling design, equipment investment, and part consistency.

As a professional manufacturer, ChinaCustomStamping embodies this principle. Our expertise in precisely calculating these forces ensures that every stamping press and die set in our facility is optimally matched for its task. This technical rigor, combined with our deep drawing capabilities, allows us to deliver complex, high-tolerance metal components reliably and efficiently for global clients. Whether you are designing a new part or optimizing an existing production process, a firm grasp of these fundamental forces is your first step toward a successful and sustainable manufacturing operation.

Understanding Bending Force, Stripper Force, and Blank Holder Force in Metal Stamping

Understanding Bending Force, Stripper Force, and Blank Holder Force in Metal Stamping

The Critical Role of Force Calculation in Stamping